Spicy Mac And Cheese

Name: Spicy Mac And Cheese
Ingredients: Kosher salt; 2 cups elbow macaroni 8 ounces cheddar cheese, half cubed and half shredded; 8 ounces colby cheese, cubed; 8 ounces pepper Jack cheese, cubed; 2 teaspoons all-purpose flour; Freshly ground black pepper; 1/2 teaspoon cayenne pepper; 1/2 teaspoon mustard powder; 1/8 teaspoon freshly grated nutmeg; 1/4 cup sour cream; 1 large egg, beaten; 1 cup heavy cream; 1 cup half-and-half; 1/4 cup finely grated onion; 4 slices sandwich bread; 1 tablespoon unsalted butter